Field of the Invention
This invention relates to the field of coding digital values arranged in a matrix, in particular in the case where this matrix is a two-dimensional matrix, representing the pixels of an image.
The main constraints of the methods of compression are, on the one hand, to reduce as much as possible, by compressing it, the volume, measures in bytes, of an initial digital file and, on the other hand, to restore a file that is as close as possible to the initial file.
Description of the Related Art
Certain methods of compression make it possible to restore exactly the initial values. This is the case with DPCM modulation. According to this method, an original value, i.e. the first value of the initial digital file, is retained, then each other value is replaced with its difference with the value before it in the initial file. The numbers corresponding to the differences are generally smaller than those corresponding to the initial values, which makes it possible to obtain a compressed file. In order to restore an initial value, it is sufficient to add the difference corresponding to the previous initial value, i.e. the various successive values are added together with the origin value. This mode of compression therefore applies to a linear sequence, i.e. extending according to a single dimension, of numerical values; the same applies for a two-dimensional matrix of numerical values, formed of lines and columns, with this mode of compression able to be applied to each line of the matrix successively; the same applies regardless of the number of dimensions of a matrix, with the mode of compression able to be applied to each line (or the equivalent in the corresponding dimension) of the matrix.
It is admitted that it is the most substantial reduction in the differences between two initial values that makes it possible to obtain the highest compression ratio possible. It is as such that DPCM modulation was introduced. Nevertheless, the rate of compression obtained with the DPCM method remains low. The idea to apply an additional compression to the file of the differences thus appears attractive. However, the errors induced by this new compression are accumulated at the same time as, during the restitution, the successive differences are added together with the original value. According to the ADPCM (Adaptive DPCM) method, these errors are partially offset by using an algorithm that is supposed to predict these errors. This method remains unsatisfactory with regards to the compression rates that it is desirable to achieve.
Moreover, in the case of a matrix representing a fixed or video image, the processing of each image is done in blocks of 8×8 pixels, and in each block, line by line, from the first to the eighth. Such a method often results in a lineage and/or a pixelization of the restored image.